a. Field of the Invention
The present invention relates to the installation of an optical fiber cable in optical fiber ducting, which may be an optical fiber microduct, and in particular to the installation in ducting of optical fiber cabling carrying optical fibers terminated at one or more multiple optical fiber connectors.
b. Related Art
Optical fiber connectors are used to join optical fibers where a connect/disconnect capability is required. The basic connector unit is a connector assembly at the end of a length of optical fiber cable, which will include a protective outer sheath around a coated optical fiber. The optical fiber will have an optical fiber core, and will often have a primary coating applied to the fiber during manufacture and a surrounding secondary polymer coating. Such a coated optical fiber is referred to as a buffered optical fiber. The buffering provides such functions as mechanical isolation, protection from physical damage and fiber identification for use during installation, but the buffered optical fiber is itself quite delicate where this is exposed for termination in a connector at the end of the protective outer sheath of the cable.
A dual connector assembly consists of two connector plugs. Due to the polishing and tuning procedures that may be incorporated into optical connector manufacturing, connectors are generally assembled onto optical fiber in a supplier's manufacturing facility. However, the assembly and polishing operations involved can be performed in the field.
SC optical fiber connectors and LC optical fiber connectors are the most common types of connectors on the market. In many data centre applications, small connectors (e.g., LC) and multi-fiber connectors (e.g., MTP) are replacing larger, older styles of connector (e.g., SC), allowing more fiber ports per unit of rack space.
Modern connectors typically use a “physical contact” polish on the fiber and ferrule end, which usually has a slightly curved, convex surface, so that when fibers are mated only the fiber cores touch, not the surrounding ferrules. Some manufacturers have several grades of polish quality, for example a regular FC connector may be designated “FC/PC” (for physical contact), while “FC/SPC” and “FC/UPC” may denote “super” and “ultra” polish qualities, respectively. Higher grades of polish give less insertion loss and lower back reflection. When a connector is terminated or assembled in the field, any damage to the polished ferrule and fiber of the connector can cause very high insertion loss.
Optical fiber cables are frequently routed in ducts. The ducts in which optical fiber cables are installed are usually made of cylindrical polyethylene tubing with a diameter ranging from typically 25 mm to 100 mm. Sometimes optical fiber cables are installed inside sub-ducts which are routed inside larger ducts, such as this type of polythene tubing or ducts formed in other materials, such as like concrete. Small sub-ducts are usually referred to as microducts and are often used to install small microduct fiber optic cables. Microducts have a size ranging from typically 3 mm to 16 mm and are may be installed as bundles inside larger ducts.
The invention described below is particularly applicable to the installation of multiple optical fiber cabling inside microducts inside microducts, for example duplex optical fiber cabling, where the internal diameter of the microduct may be less than the maximum width of the multiple optical fiber connector to be fitted at the terminated end of the cabling.
In the context of the present invention, the term “duplex” is used to mean “dual”, “two” or “twin” fibers or assemblies, rather than a single fiber used for bi-directional transmission. Similarly, the term “multiple” is used to mean “two or more”.
As will become apparent, the invention may, however, also be useful when installing duplex or multiple optical fiber cabling inside larger ducts, and therefore the terms “duct” and “ducting” as used in relation to the invention apply both to microducts and to larger ducts for routing optical fiber cables without microducting.
One common form of optical fiber cable is the duplex optical fiber cable, with one fiber being used for transmitting (Tx) and the other fiber being used for receiving (Rx). For convenience, the duplex optical fibers are terminated within a duplex optical fiber connector unit having a joined pair of side-by-side connectors, each with a termination ferrule for terminating one of the optical fiber cores. The duplex optical fibers may be held within a cable having a single outer sheath or in a cable having a pair of joined outer sheaths. Multiple duplex pairs may also be provided within one cable, each duplex pair being for termination in one duplex connector. The invention is applicable to any such cable having at least one pair of optical fibers for termination in a duplex optical fiber connector, where the optical fiber cable is to be installed inside an optical fiber duct.
The installation of fiber optic cables in ducts can be done by pulling, in which a pre-installed thread or wire inside the duct is used to pull the connector end of an optical fiber cable down a length of the ducting. Usually, the cable being inserted into the near end of the duct is mechanically pushed at the same time. Alternatively, a technique called cable jetting (also called cable blowing), can be used, together with cable pushing. In cable jetting, high pressure air is blown down the duct and the flow of air pulls along the inserted cable until the cable exits the far end of the duct.
It is not common to install a terminated fiber cable into already occupied duct because of the danger of entanglement and damage. The space inside the duct is normally limited by the customer's desire to use the smallest possible duct. Installing a duplex connector in a duct would be more difficult than installing a simplex connector owing to the larger dimensions of the duplex connector at the end of the cable. Even in the case of the smaller LC optical fiber duplex connector, there may be inadequate clearance inside the duct for the duplex connector when pulling or jetting the cable down the length of ducting. There is also the problem that the duplex connector, or the terminated optical fiber ferrules within the connector, may have to be protected from damage or contamination during installation within the ducting, which may require additional protection to be fitted to or around the duplex connector, thereby potentially increasing the exterior dimensions of the connector and making it even more difficult, or impossible, to fit the duplex connector into the available space within the ducting.
It is an object of the invention to address these problems and to provide a more convenient apparatus and method for installing a multiple optical fiber connector when the optical fiber cable is to be routed in a duct for optical fiber cabling.